Abstract
Folding of the myosin head often requires the joint actions of Hsp90 and a dedicated UNC45, Cro1, She4 (UCS) domain-containing cochaperone protein. Relatively weak sequence conservation exists between the single UCS protein of simple eukaryotes (She4 in budding yeast) and the two UCS proteins of higher organisms (the general cell and smooth muscle UNC45s; UNC45-GC and UNC45-SM respectively). In vertebrates, UNC45-GC facilitates cytoskeletal function whereas the 55% identical UNC45-SM assists in the assembly of the contractile apparatus of cardiac and skeletal muscles. UNC45-SM, unlike UNC45-GC, shares with yeast She4 an IDSL sequence motif known to be a site of in vivo serine phosphorylation in yeast. Investigating this further, we found that both a non-phosphorylatable (S18A) and a phosphomimetic (S18E) mutant form of She4 could rescue the type 1 myosin localisation and endocytosis defects of the yeast she4Δ mutant at 39 °C. Nevertheless, at higher temperature (45 °C), only She4 (S18A), not She4(S18E), could substantially rescue the cell lysis defect of she4Δ mutant cells. In the yeast two-hybrid system, the non-phosphorylatable S18A and S251A mutant forms of She4 and UNC45-SM still displayed the stress-enhanced in vivo interaction with Hsp90 seen with the wild-type She4 and UNC45-SM. Such high-temperature enforcement to interaction was though lost with the phosphomimetic mutant forms (She4(S18E) and UNC45-SM (S251E)), an indication that phosphorylation might suppress these increases in She4/Hsp90 and UNC45-SM/Hsp90 interaction with stress.
Highlights
Vertebrates have two forms of cytosolic Hsp90, Hsp90α and Hsp90β
As described below, Hsp90α and Hsp90β differ in their association with UNC45, Cro1 and She4 (UCS) proteins, cochaperones that cooperate with Hsp90 during the folding of the myosin head
The functionality of She4 in yeast was studied as its ability, when expressed in cells of the she4Δ mutant, to rescue (i) the defective localisation of the class I myosin Myo5—observed as a functional Myo5-green fluorescent protein (GFP) fusion—at 39 °C, (ii) the defective localisation of the fluorescent dye FM4-64 to the vacuolar membrane, at 39 °C and (iii) cell survival at 45 °C
Summary
Vertebrates have two forms of cytosolic Hsp, Hsp90α and Hsp90β. The stress-induced isoform is Hsp90α whereas Hsp90β is more usually constitutively expressed and seems to be associated with development, long-term cell adaptation and evolution (Sreedhar et al 2004). Hsp90β is essential for embryonic development, while the loss of Hsp90α is fully compatible with viability though it causes a block to spermatogenesis (Grad et al 2011). Zebrafish studies have identified Hsp90α as being highly expressed in striated muscle while Hsp90β predominates in other tissues (Krone et al 2003). As described below, Hsp90α and Hsp90β differ in their association with UNC45, Cro and She (UCS) proteins, cochaperones that cooperate with Hsp during the folding of the myosin head
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